The Axl receptor tyrosine kinase was identified as a protein encoded by a transforming gene from primary human myeloid leukaemia cells by DNA-mediated transformation of NIH 3T3 cells. Axl is the founding member of a family of related receptors that includes Eyk, encoded by a chicken proto-oncogene originally described as a retroviral transforming gene, and c-Mer, encoded by a human proto-oncogene expressed in neoplastic B- and T-cell lines. The transforming activity of Axl demonstrates that the receptor can drive cellular proliferation. The function of Axl in non-transformed cells and tissues is unknown, but may involve the stimulation of cell proliferation in response to an appropriate signal, namely a ligand that activates the receptor. We report here the purification of an Axl stimulatory factor, and its identification as the product of growth-arrest-specific gene 6 (ref. 6). This is, to our knowledge, the first description of a ligand for the Axl family of receptors.

Several receptor tyrosine kinases generate soluble ligand binding domains either by differential splicing resulting in a truncated RNA transcript, or by proteolytic cleavage. Although the exact role in vivo of these soluble extracellular domains is unclear, proteolysis may function to down-regulate the receptor, and soluble extracellular domains (ECD) may compete with the intact receptor binding to ligand. Axl is a member of a new class of receptor tyrosine kinases characterized by an ECD resembling cell adhesion molecules and unique sequences in the kinase domain. In addition, Axl is transforming in both fibroblast and hematopoietic cells, and appears to be involved in mesenchymal development. We now find that Axl is post-translationally processed by cleavage in a 14 amino acid region immediately NH2-terminal to the transmembrane domain resulting in a soluble ECD and a membrane bound kinase domain. The sequence of this putative cleavage site shares no homology with recognition sites of known proteases. Characterization of this proteolytic processing shows that it does not require protein synthesis or transport but is augmented by phorbol ester treatment. Since the cleavage of Axl enhances turnover of the kinase on the cell surface, we suggest that proteolytic processing down-regulates Axl kinase activity.

The oxidative stress hypothesis of aging predicts that a reduction in the generation of mitochondrial reactive oxygen species (ROS) will decrease oxidative damage and extend life span. Increasing mitochondrial proton leak-dependent state 4 respiration by increasing mitochondrial uncoupling is an intervention postulated to decrease mitochondrial ROS production. When human UCP2 (hUCP2) is targeted to the mitochondria of adult fly neurons, we find an increase in state 4 respiration, a decrease in ROS production, a decrease in oxidative damage, heightened resistance to the free radical generator paraquat, and an extension in life span without compromising fertility or physical activity. Our results demonstrate that neuronal-specific expression of hUCP2 in adult flies decreases cellular oxidative damage and is sufficient to extend life span.

Atherosclerosis and arterial restenosis are disease processes involving the accumulation of vascular smooth muscle cells following vascular injury. Key events leading to these processes are migration and proliferation of these cells. Here, we demonstrate that GAS6, encoded by the growth arrest-specific gene 6, induces a directed migration (chemotaxis) of both rat and human primary vascular smooth muscle cells while showing only marginal mitogenic potential in human vascular smooth muscle cells. GAS6 stimulation induces Axl autophosphorylation in human vascular smooth muscle cells, indicating that specific GAS6-Axl interactions may be associated with GAS6-directed chemotaxis. To test this hypothesis, vascular smooth muscle cells overexpressing Axl were generated by gene transfer and assessed for their ability to migrate along a GAS6 gradient. These Axl overexpressors exhibited 2-5-fold increased sensitivity to GAS6-induced chemotaxis. Furthermore, vascular smooth muscle cells expressing the kinase dead mutant of Axl or exposure to the soluble Axl extracellular domain showed attenuated GAS6-induced migration. Taken together, these results suggest that GAS6 is a novel chemoattractant that induces Axl-mediated migration of vascular smooth muscle cells. The separation of mitogenesis from migration provided by this study may enhance the molecular dissection of cell migration in vascular damage.Atherosclerosis and arterial restenosis is a consequence of accumulation of connective tissue in conjunction with proliferation and directed migration of vascular smooth muscle cells (VSMC) 1 (1). To evaluate potential in vivo effects of proliferation and directed migration of VSMC following treatment of agonists or growth factors, primary, cultured VSMC have proven to be an excellent in vitro model system.In cultured rat VSMC, GAS6, encoded by the growth arrestspecific gene 6 (gas6), was identified and characterized as an important growth-potentiating factor whose expression is upregulated after serum starvation (2-4). GAS6 possesses a 44% sequence identity with protein S, an anti-coagulation factor (4). In quiescent VSMC, GAS6 stimulation specifically potentiates proliferation induced by Ca 2ϩ -mobilizing receptors indicating that GAS6 may be involved in regulating signaling pathways mediated by heterotrimeric guanine nucleotide-binding (G) protein-coupled receptors. GAS6 alone, however, is able to prevent growth arrest-induced apoptosis in these cells (5). We have identified GAS6 to be the ligand for Axl, a member of a tyrosine kinase receptor family whose extracellular domains resemble cell adhesion molecules (6 -13). It was subsequently demonstrated that GAS6 is also a ligand for Sky, an Axlrelated receptor tyrosine kinase (14 -16). In addition, it was recently shown that GAS6 may be the ligand for another Axl family member, Mer, although the affinity of the GAS6-Mer interaction is much lower than that for Axl and Sky (17). In rat VSMC, a high affinity, specific binding site for GAS6 was characterized and the molecular weig...

The neuroendocrine architecture and insulin/insulin-like signaling (IIS) events in Drosophila are remarkably conserved. As IIS pathway governs growth and development, metabolism, reproduction, stress response, and longevity; temporal, spatial, and nutrient regulation of dilps encoding Drosophila insulin-like peptides (DILPs) provides potential mechanisms in modulating IIS. Of eight DILPs (DILP1–8) identified, recent studies have furthered our understanding of physiological roles of DILP2, DILP3, DILP5, and DILP6 in metabolism, aging, and responses to dietary restriction (DR), which will be the focus of this review. While the DILP producing IPCs of the brain secrete DILP2, 3, and 5, fat body produces DILP6. Identification of factors that influence dilp expression and DILP secretion has provided insight into the intricate regulatory mechanisms underlying transcriptional regulation of those genes and the activity of each peptide. Studies involving loss-of-function dilp mutations have defined the roles of DILP2 and DILP6 in carbohydrate and lipid metabolism, respectively. While DILP3 has been implicated to modulate lipid metabolism, a metabolic role for DILP5 is yet to be determined. Loss of dilp2 or adult fat body specific expression of dilp6 has been shown to extend lifespan, establishing their roles in longevity regulation. The exact role of DILP3 in aging awaits further clarification. While DILP5 has been shown associated with DR-mediated lifespan extension, contradictory evidence that precludes a direct involvement of DILP5 in DR exists. This review highlights recent findings on the importance of conserved DILPs in metabolic homeostasis, DR, and aging, providing strong evidence for the use of DILPs in modeling metabolic disorders such as diabetes and hyperinsulinemia in the fly that could further our understanding of the underlying processes and identify therapeutic strategies to treat them.